The catalysts used most frequently in oxygen reduction reaction in a fuel cell are largely formed by platinum (Pt)-based noble metals. Such platinum catalysts are produced in a very small amount all over the world, and thus are highly expensive. Due to the problem of such high price of platinum, commercialization of fuel cells are delayed. To solve such a problem, active studies have been conducted recently about platinum alloy catalysts in order to reduce the use of platinum. In the case of a platinum alloy catalyst, metals other than platinum are used in a predetermined ratio and the amount of platinum may be reduced, resulting in a decrease in price of such a catalyst. In addition, such alloying causes a change in atomic structure, resulting in an increase in activity.
As an alloy catalyst for fuel cells, active studies have been conducted about PtM including platinum alloyed with a transition metal (M=Ni, Co, Fe, Cr, V, Ti) and having a face-centered cubic lattice structure. In general, a platinum catalyst supported on carbon is obtained by a precursor deposition process in which a metal precursor is deposited on a platinum catalyst supported on carbon, followed by heat treatment. After a metal precursor is deposited on a platinum catalyst supported on carbon, heat treatment is carried out at 700-1200° C. while a gaseous reducing agent such as hydrogen is allowed to flow therethrough, thereby providing a platinum alloy catalyst. Although such heat treatment carried out at high temperature increases the alloying degree of a catalyst and catalytic activity, it causes an increase in platinum particle size and agglomeration of particles, resulting in a decrease in catalytically active area.
Therefore, more recently, some studies have been conducted about production of a platinum alloy catalyst supported on carbon without heat treatment at high temperature. Xiong and coworkers prepared a platinum alloy catalyst using a carbonyl complex process (Electrochemistry communications, 10 (2006) 1671-1676), while Santos and coworkers prepared a Pt—Ni alloy catalyst by a microemulsion process using a surfactant. In addition, Li and coworkers prepared a Pt—Fe alloy catalyst by using a polyol process (Electrochimica Acta 49 (2004) 1045-1055). Further, Su and coworkers prepared an alloy catalyst at 400° C. using a hydrogen reduction reaction (Journal of Power Sources 205, (2012) 136-144). The above-mentioned processes successfully inhibited particle size growth after alloying. However, they have problems in that it is difficult to control the ratio of metals for use in alloying and they provide a low alloying degree. In addition, a platinum alloy catalyst having a low alloying degree has a large amount of transition metals present on the surface thereof. Such transition metals present on the surface easily dissolved out in the acidic environment of a fuel cell, thereby causing degradation of the durability of a fuel cell.
Therefore, there is a need for forming a layer having a high alloying degree, i.e., high platinum density, so-called a Pt skin in a platinum alloy catalyst supported on carbon, as well as for a method for inhibiting particle size growth while carrying out heat treatment at high temperature.
To solve the above problems, the inventors of the present disclosure have developed a method for inhibiting the growth of a platinum alloy catalyst during high-temperature heat treatment by introducing polypyrrole (Ppy) as a capping agent (Korean Patent Publication No. 10-1231006). Although the method inhibited the growth of platinum alloy catalyst particles despite high-temperature heat treatment, it has a disadvantage in that additional equipment is required because of Ppy coating carried out at a low temperature of 4° C. Moreover, due to the large thickness of a Ppy coating layer, transition metal particles such as Ni or Co diffuse slowly into platinum particles during heat treatment, and the method cannot provide a perfect core-shell structure and has a limitation in alloying degree.